linux-kernel - Re: [RFC bpf-next 0/4] bpf: Speed up symbol resolving in kprobe multi link

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Date:   Fri, 8 Apr 2022 16:29:22 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Jiri Olsa <jolsa@...nel.org>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Masami Hiramatsu <mhiramat@...nel.org>, netdev@...r.kernel.org,
        bpf@...r.kernel.org, lkml <linux-kernel@...r.kernel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>
Subject: Re: [RFC bpf-next 0/4] bpf: Speed up symbol resolving in kprobe
 multi link

On Thu, Apr 07, 2022 at 02:52:20PM +0200, Jiri Olsa wrote:
> hi,
> sending additional fix for symbol resolving in kprobe multi link
> requested by Alexei and Andrii [1].
> 
> This speeds up bpftrace kprobe attachment, when using pure symbols
> (3344 symbols) to attach:
> 
> Before:
> 
>   # perf stat -r 5 -e cycles ./src/bpftrace -e 'kprobe:x* {  } i:ms:1 { exit(); }'
>   ...
>   6.5681 +- 0.0225 seconds time elapsed  ( +-  0.34% )
> 
> After:
> 
>   # perf stat -r 5 -e cycles ./src/bpftrace -e 'kprobe:x* {  } i:ms:1 { exit(); }'
>   ...
>   0.5661 +- 0.0275 seconds time elapsed  ( +-  4.85% )
> 
> 
> There are 2 reasons I'm sending this as RFC though..
> 
>   - I added test that meassures attachment speed on all possible functions
>     from available_filter_functions, which is 48712 functions on my setup.
>     The attach/detach speed for that is under 2 seconds and the test will
>     fail if it's bigger than that.. which might fail on different setups
>     or loaded machine.. I'm not sure what's the best solution yet, separate
>     bench application perhaps?

are you saying there is a bug in the code that you're still debugging?
or just worried about time?

I think it's better for it to be a part of selftest.
CI will take extra 2 seconds to run.
That's fine. It's a good stress test.

>   - copy_user_syms function potentially allocates lot of memory (~6MB in my
>     tests with attaching ~48k functions). I haven't seen this to fail yet,
>     but it might need to be changed to allocate memory gradually if needed,
>     do we care? ;-)

replied in the other email.

Thanks for working on this!